It's hard to offer any suggestions without knowing anything about the sort of holiday you're looking for. Assuming you are landing at Bangkok, you could have a few days there to see the Royal Palace, temples and other sights; after which - as long as the traffic in Bangkok hadn't got you - head south for the beaches and islands, such as Phuket, Ko Samui and Langkawi. On the other hand, if you are more into culture and archeological sites, then you may wish to travel north to Ayutthaia (the old capital), Lopburi (with a famous monkey temple), Sukkothai (with some stunningly beautiful temples and Buddha statues) and the walled city of Chiang Mai - as you may guess, this would be more my choice. That said, in a month you should be able to combine both sightseeing and sun-bathing, if you wish.